The present invention relates to data storage generally and more particularly to holographic data storage.
Holographic data storage typically achieves high data densities by overlapping many data-bearing holograms in the same recording volume. However, many proposed holographic multiplexing techniques (e.g., shift multiplexing, fractal multiplexing, and correlation multiplexing) result in partially overlapping holograms or groups of holograms. In this case, the device controller is faced with the challenge of formatting the media with intermingled data records. ([1] Coufal, H. T., Psaltis, D., and Sincerbox, G. T. (Eds.), Holographic Data Storage, Springer-Verlag, 2000; [2] Mok, F. H., et. al., xe2x80x9cSystem metric for holographic memory systems,xe2x80x9d Optics Letters, v. 21, n. 12, Jun. 15, 1996, pp. 896-898.)
In particular, a major constraint of the underlying recording technology is the typical requirement for approximately uniform grating strength across each hologram. According to one approach, an entire hologram is uniformly written on a holographic medium that has been uniformly exposed by previous read and write operations. Alternatively, hologram recording signals may be equalized across their spatial extent by varying intensity or exposure time to account for the non-uniform response of the underlying medium; however, this can lead to complex exposure scheduling that may require integrated measurements.
Additional disadvantages result when the medium has been only partially exposed by recording operations because the remaining capacity is likely to be lost or diminished by subsequent reading operations. Since the holographic recording medium is typically photoreactive in response to the readout (reference) signal, the medium will lose overall recording capacity when the host performs read operations on recorded data that are in partially exposed regions.
Thus, there is a need for recording holograms in a way that maintains uniform exposure and minimizes the amount of partially exposed media.
In one embodiment of the present invention, a method of writing holograms includes: numbering sections in sequence, where each section has a spatial projection onto a holographic medium; and writing at least one hologram in each of the sections in sequence. Writing in two sections with adjacent spatial projections provides a substantially uniform exposure level for a sequentially subsequent section with a spatial projection that overlaps the two sections.
According to one aspect, one or more holograms can be tested to determine the available dynamic range for recording additional holograms. For example, a test hologram can be measured to determine its efficiency from which an available dynamic range can be estimated for the corresponding location in the holographic medium. An exposure schedule at that location then can be correspondingly adjusted. Similarly, multiple test holograms can be measured to determine available dynamic ranges and exposure schedules at multiple locations.
According to another aspect, additional sections of the holographic medium can be exposed just for the purpose of maintaining a substantially uniform exposure level when sequentially writing holograms in the numbered sections. For example a bookend of additional sections near a boundary of a shelf of the numbered sections can be exposed to provide a substantially uniform exposure level when writing holograms in the shelf in sequence near the boundary. Each additional section similarly has a spatial projection onto the holographic medium and overlaps at least one section of the shelf.
In another embodiment of the present invention, a holographic medium tangibly embodies holograms in a sectional format. The format includes: multiple sections, where each section has a spatial projection onto the holographic medium and each section has a sequence number for storing holograms in sections; and at least one hologram in at least one section, where each section has an exposure level corresponding to holograms therein. Two sections with adjacent spatial projections provide a substantially uniform exposure level for a sequentially subsequent section with a spatial projection that overlaps the two sections. This embodiment may further include the aspects described above.
The present invention enables holographic recording in a way that maintains uniform exposure and minimizes the amount of partially exposed media.